Google Nest Learning Thermostat (4th gen) wiring for 3 wires, heat + fan

The Google Nest Learning Thermostat (4th gen) works with this setup, but you’ll need an adapter or extra wire (details below).

Three wires on R, W and G control a furnace’s heat and blower but not AC, and there is no C wire. Some smart thermostats cope; others need extra wire.

1Snap a photo of your old thermostat’s wires (optional)

📷 Upload or take a photo

AI reads the terminal letters and wire colors, then you double-check them. Photos are not saved.

2Tick every terminal that has a wire

Pull the faceplate off your old thermostat. Use the letter printed next to each wire, not its color. Skip short jumper wires between two terminals.

More labels (W3, X, X2, K, PEK, L, S, T, U, HUM, DEHUM)

3A few quick questions

Do you have a heat pump?

YesNoNot sure

A heat pump is an outdoor unit that both heats and cools. Heat pump thermostats usually have an “EM HEAT” or “AUX” setting.

Do any of these apply? Thick wires joined with wire nuts, 120/240 V labels, or electric baseboard heaters (line voltage) Gas wall/floor heater or fireplace that runs without household power (millivolt) Numbered terminals (1, 2, 3, 4 or A, B, C, D), e.g. Carrier Infinity or Lennox iComfort

4Your wiring

Your system: 1-stage heat, fan, no C wire · 3 wires

Which smart thermostats work with your wires?

Google Nest Learning Thermostat (4th gen)

Works with an adapter

Google says heat-only, cool-only and heat pump systems without a C wire need a C wire or the Nest Power Connector.

Wire	Color	Job	Connect to	Note
R	red	24 V power	Rc or Rh
W	white	Heat, stage 1	W
G	green	Fan	G

Install steps

Turn off power to the furnace/air handler (switch or breaker) and the AC condenser. Confirm the old thermostat is dead.
Photograph the old wiring, then label each wire with the letter it came off. Ignore short jumpers.
Install the adapter at your furnace or air handler following its instructions, with power still off.
Connect each wire to the Nest Learning (4th gen) terminal shown above. Cap unused wires one at a time.
Restore power and finish setup in the app using the settings below.
Test heat, cool and fan. If a breaker trips or equipment runs nonstop, power off and call a pro.

App settings to choose

Single R wire: the Nest app tells you whether to use Rc or Rh.

What you need

Google Nest Learning Thermostat (4th gen)The thermostat itselfCheck price
Google Nest Power ConnectorSteady power when there’s no C wireCheck price

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Confirm against the official Google Nest install guide. The app also checks your wiring during setup.

Frequently asked questions

Where does each wire go on the Nest Learning (4th gen)?

R → Rc or Rh; W → W; G → G.

Do I need a C wire for the Nest Learning (4th gen)?

Google says heat-only, cool-only and heat pump systems without a C wire need a C wire or the Nest Power Connector.

Other thermostats for this setup

Nest Learning (4th gen): other setups